1. Field of the Invention
This invention relates to metering apparatus for aspirating discrete liquid volumes, either for subsequent dispensing or passing to an analytical system, wherein contamination between successively aspirated volumes is avoided.
2. Description of the Prior Art
In prior art metering apparatus, contamination between successively aspirated liquid volumes has been a major problem. Such contamination can result, for example, from residue remaining on the probe surfaces from a previously aspirated liquid volume. The avoidance of contamination is of particular concern where segments of sample liquids are successively aspirated, in precise volumes, for the analysis of different constituents of interest, for example, as in continuous-flow analytical systems of the type described in the Skeggs et al U.S. Pat. No. 3,241,432, issued on Mar. 22, 1966 and in the Smythe et al U.S. Pat. No. 3,479,141, issued on Nov. 18, 1969. In such systems, contamination has been significantly reduced by aspirating a segment wash liquid between successive sample segments, adjacent sample segments being separated by a sequence of air-wash liquid-air segments. To this end, the aspirating probe is immersed into a wash liquid reservoir between successive sample immersions, which serves to remove contaminants from both the interior and peripheral probe surfaces. Also, U.S. Pat. No. 3,479,141 teaches that contamination between successive sample liquids in continuous-flow analytical systems is very significantly reduced by introducing a liquid segment, e.g., silicone, which is immiscible with the aqueous sample segments and preferentially wets the interior surfaces of the probe and conduit system. In such event, the successive sample segments are, in effect, encased within the immiscible fluid, and not in contact with the interior conduit surfaces.
Also, in the case of metering apparatus of the dispenser-type, the aspirating probe is immersed into successive liquids, either samples or reagents, which are aspirated and dispensed, sequentially and in precise volumes. Such dispensing is effected by the use of a pilot fluid, which serves to "back-flush" contaminants from the interior surface of the probe system. Again, possible contamination resulting from residues on the peripheral probe surfaces has been avoided by immersing the probe into a wash liquid reservoir, and aspirating and dispensing such liquid to remove contaminants from both the interior and peripheral probe surfaces.
The removal of contaminants from the peripheral probe surfaces has required that the probe movement be other than rectilinearly, that is, to transport the probe from an aspirating station to a wash station. Also, it is necessary that the probe surfaces, both interior and peripheral, be washed between successive aspirations. This need to actively wash the probe surfaces necessarily reduces the rate at which precise liquid volumes can be aspirated or dispensed, and, also, requires a more complicated probe-driving mechanism.